A deep understanding of visual system function will require both a knowledge of the physiological properties of individual neurons and a detailed knowledge of the patterns by which these units are interconnected. Recently anatomical methods have been extraordinarily successful in determining which regions and subregions of the brain are interconnected and have provided, in essence, excellent "block diagrams." So far, however, very little is known about the detailed "wiring" of any region of the visual system. Although it is known, for example, that synapses from the lateral geniculate terminate in layers I, IV, and VI of cortical area 17, it is not known which of the many cell types there receive the input, nor whether the input is on the cell bodies, proximal or distal dendrites -- all questions of grave physiological consequence. We propose to answer such questions by constructing with serial section cinematography detailed "wiring diagrams" of the cat retina, visual cortex (layer IV of area 17), and superior colliculus (superficial gray). Our procedure is to photograph long series of thin sections in the electron microscope. Successive negatives are aligned and rephotographed to produce a high resolution 35 mm "movie." The detailed synaptic connections are reconstructed by viewing the movie frame-by-frame through a microscope. Reconstructions are carried out on experimental material with particular cells and afferents labelled by orthograde degeneration, horse radish peroxidase, and radioisotopes. The resulting wiring diagrams should provide a firmer basis for understanding the physiology of the neurons and also the changes in synaptic organization that occur during development and in pathologic states.